The term "polyphenylene ether resin" is well known in the art as defining a class of thermoplastic polymers which possess a number of outstanding properties. They are useful for many conventional applications requiring high temperature resistance, including the formation of film, fiber and molded articles. Methods of their preparation are disclosed, for example, in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875 and in Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, which are incorporated herein by reference.
It is known that when a polyphenylene ether resin is combined with a styrene resin, the resulting composition provides properties which are better than those of either resin alone. See, e.g., Cizek, U.S. Pat. No. 3,383,435, the disclosure of which is incorporated herein by reference.
The polyphenylene ether resins and composites of a polyphenylene ether resin and a styrene resin are characterized by their normally high heat deflection temperature, as well as their high melt viscosities. The property heat deflection temperature, also known in the art as the "heat distortion temperature," is a measure of the ability of a test specimen of given dimensions to resist deformation under load at elevated temperatures. High heat deflection temperatures are generally desirable in articles molded from thermoplastic compositions. On the other hand, a high melt viscosity in the molten resin often makes processing more difficult and is therefore undesirable in many molding applications.
Previous efforts have been made to improve the melt flow by reducing the melt viscosity of polyphenylene ether resin alone and in combination with styrene resins. Improvements in the melt flow have usually been at the cost of significant reductions in the heat deflection temperature and other desirable properties, however. For instance, it is disclosed in British Pat. No. 1,344,729 that the melt viscosity of a polyphenylene ether resin is lowered by the addition of certain high melting hydrocarbon resins derived from coal-tar naphthas, e.g., styrene resins, coumarone resins, indene resins, cyclopentadiene resins, and others. The patentees state that these compositions are also characterized by reduced heat distortion temperatures, i.e., heat deflection temperature, in almost direct proportion to the amount of the high melting hydrocarbon resin present (page 3, first column, lines 11-15).
It has now been discovered that when certain high softening point resinous materials are added to compositions comprising a polyphenylene ether resin in admixture with a styrene resin, the melt flow is improved without any appreciable loss in the heat deflection temperature level in the composition after molding, and without sacrificing other desirable physical properties to any significant extent. This discovery is surprising in view of the teachings in the aforementioned British patent to the effect that the heat deflection temperature of a polyphenylene ether resin alone undergoes a significant decrease when certain high melting resins are added. Moreover, the high softening point resinous additives of this invention are brittle, low molecular weight substances which are generally regarded as being suitable only in non-load-bearing applications, such as printing inks, pigment wetting, tackifiers, adhesives, and the like, and their effectiveness in reducing the melt viscosity without decreasing the heat deflection temperature in this invention is all the more unexpected.